The present invention relates to ghost removal apparatus which remove waveform distortion or ghost and which are used in television receivers and other various types of image apparatus which handle television signals. In particular, the present invention has as an object the provision of a ghost removal apparatus which can exclude the influence of random noise when there is ghost removal, and which can favorably remove minute ghost components.
In recent years, with the shift to high-definition television broadcasting, television image signals have been inserted into a reference signal (GCR signal) for ghost cancellation in an attempt to improve the image quality of television broadcasts. On the side of the receiver, the reference signals for ghost cancellation are extracted and those signals are used as the basis for ghost removal. These GCR signals are disclosed in detail in Japanese Patent Application No. 1-69179/1989 (Laid-Open No. 2-290398/1990) and U.S. patent application Ser. No. 07/783,826, and hence the details of this will be omitted here.
FIG. 1 shows a conventional example of a known ghost removal apparatus which has a construction and operation on the basis of the principle described above.
A conventional ghost removal apparatus 1 comprises, as shown in FIG. 1, an analog/digital (A/D) conversion circuit 2 which converts inputted analog image signals S.sub.IN into digital signals, a transversal filter 3 which removes a ghost component from the converted digital signals by performing a calculated weighting for a cancellation of ghost, a digital/analog (D/A) conversion circuit 4 which converts the output of the transversal filter 3 into analog signals again to output the analog signals as image signals S.sub.OUT, a timing generator circuit 5 which generates various signals such as vertical sync signals, horizontal sync signals and waveform extraction pulses from the inputted image signals S.sub.IN, a waveform extraction circuit 6 which extracts a signal component of a required constant cycle (such as a single scan line portion) including the GCR signals from the output transversal filter 3, and a calculation processing circuit 7 which detects and determines as to whether or not a tap gain of the transversal filter 3 should be changed on the basis of the outputs of the timing generator circuit 5 and the waveform extraction circuit 6, and which calculates the tap gain when it should be changed.
Even though there is eliminated a description by using the figures, the calculation processing circuit 7 comprises a waveform check circuit for checking a waveform and timing of the waveform extraction circuit 6, a sync addition circuit for converting an output of the waveform extraction circuit 6 into a reference waveform, an ideal reference waveform creation circuit for generating an ideal reference waveform, a waveform comparison circuit for comparing both of the outputs from the sync addition circuit and the ideal reference waveform creation circuit, an estimation function calculating circuit for calculating an estimation function on the basis of an output of the waveform comparison circuit, a minimum value storage circuit for storing a minimum value of the estimation function, a comparison processing circuit for comparing the estimation function which is calculated by the estimation function calculation circuit and the minimum value of the estimation function which is stored in the minimum value storage circuit and for determining as to whether or not a present estimation function which is a condition for renewal of a tap gain coefficient, is less than a minimum value of the past estimation function, a cumulative addition circuit for performing a cumulative addition averaging of error signal trains on the basis of an output of the waveform comparison circuit, a multiplication ratio setting circuit for determining a multiplication ratio for renewal of the tap gain coefficient on the basis of both of the outputs of the comparison processing circuit and the cumulative addition circuit, and a tap gain setting circuit for determining the tap gain coefficient of the transversal filter 3 on the basis of the multiplication ratio determined by the multiplication ratio setting circuit.
The conventional ghost removal apparatus described above, has two important comparison and determination operations in order to renew the tap gain coefficient of the transversal filter 3. Namely, one is a comparison and determination operation for successively renewing the estimation function by means of the comparison processing circuit, and the other is a comparison and determination operation for comparing the cumulative addition average with the ghost detection coefficient by means of the multiplication ratio setting circuit. In the latter case, when the cumulative addition average is larger than the ghost detection coefficient which has been set to a constant value, it is judged that a ghost component is contained in the image signals.
Accordingly, there is the following first problem when the influence of the ghost detection coefficient is set to a larger value in order to avoid the influence of the random noise component. Namely, it is difficult for the ghost removal apparatus to detect a residual ghost component such as a minute ghost component so that the removal of residual ghost becomes more difficult because the cumulative addition average, that is, the residual ghost component becomes smaller with the advance of ghost removal processing.
On the other hand, there is the following second problem when the ghost detection coefficient is set smaller so as to facilitate detection of the residual ghost component. Namely, at the start of ghost removal processing, when there is a low S/N (signal to noise) ratio, the random noise component included in the cumulative addition average is erroneously detected by the ghost removal apparatus as a ghost component, and an unnecessary tap gain is set with respect to the transversal filter 3, and there is a wrong phenomenon such as erroneous operation or a divergence.